In recent years, solar cell has attracted increasingly more attention as a practical new energy. It is a semiconductor device that converts solar energy to electrical energy using the photovoltaic effect, which greatly reduces the dependence of human production and life on coal, petroleum and natural gas, and becomes one of the most effective methods utilizing green energy sources. Among all new energies, solar energy is one of the most ideal renewal energies. Thorough development and utilization of solar energy have become an energy strategic decision made by all governments in the world for sustainable development. Over recent years, with the development of concentrating photovoltaic technology (CPV), an increasing attention has been paid to the III-V compound semiconductor solar cell due to its high photoelectric conversion efficiency.
For the III-V compound semiconductor field, the epitaxial growth of a lattice matching GaInP/GaAs/Ge triple junction solar cell on a Ge substrate is a relatively mature technology, for its conversion efficiency is as high as 41%. In the lattice matching GaInP/GaAs/Ge three junction solar cell, with the cell band gap of the Ge substrate at 0.66 eV and AM1.5D, the photocurrent density Jph is about 27.0 mA/cm2, which is twice as much as the photocurrent of the GaInP/GaAs/Ge triple-junction stacked solar cell. However, mismatching current may reduce efficiency of the Ge substrate cell since the working current of the multi junction solar cell is determined by the one with the minimum short-circuit current among the subcells. One of the effective methods to solve this problem right now is to further insert an InGaNAs subcell that has a lattice matching the Ge substrate and the GaAs material and has a band gap of about 1.0 eV between the center cell and the substrate cell, thereby obtaining an InGaP/GaAs/InGaNAs/Ge four-junction solar cell. This can make the current more matching than in the three junction solar cell. Moreover, the increase of the junction number can make the Sun spectrum to be divided in higher resolution and increase the efficiency. Due to the very low solid solubility of N atoms in the InGaAs material, however, there is a high defect density and overly short life and diffusion length of photo-induced carriers, which makes it difficult to meet the high quality requirement by solar cells. As a result, the efficiency of the InGaP/GaAs/InGaNAs/Ge four-junction solar cell is far lower than that of the three junction solar cell instead. Since the crystal quality of InGaNAs is limited by the material itself, the InGaP/GaAs/InGaNAs/Ge four-junction solar cell can be successful only when breakthroughs are achieved on material growth. Therefore, the development of a novel four-junction solar cell device that can take the place of the InGaP/GaAs/InGaNAs/Ge four-junction solar cell has become the key for further improvement of the III-V solar cell efficiency.